The core in the pressure vessel of a pressurized-water reactor, for example, is formed by a large number of fuel assemblies. Each assembly comprises a fuel assembly supporting structure holding a bundle of fuel rods laterally interspaced by distances determined by the nuclear physics involved.
Pressurized water circulates through the vessel and a reactor coolant system comprising one or more pipe loops each including a steam generator and coolant pump for maintaining the circulation. A loss-of-coolant accident, such as a serious break in any of the loops, results in a pressure drop in the pressure vessel, the coolant then discharging from the vessel at high velocity. In the event of such an accident, the reactor protective system causes control rods to drop into the core to effect a scram, while activating an emergency core cooling system which injects emergency water into the pressure vessel to flood the core and thereafter circulate the water enough to control the after-heating of the core. The emergency core cooling system is designed to function adequately if the interspacing between the fuel rods of each assembly, remains undisturbed, but difficulties occur if this interspacing is dearranged.
Each fuel assembly is formed by a fuel assembly supporting structure comprising vertically interspaced top and bottom end pieces which each comprise a flat horizontal plate which is perforated to pass the coolant and having a plurality of holes through which the end portions of vertical control rod guide tubes are inserted and fastened to the end pieces. Between the end pieces the guide tubes support fuel rod spacer grids, and the bundle of fuel rods is positioned with their designed interspacing by these grids. The guide tubes also function to receive and guide the control rods which can be inserted or raised as required, while guided by the guide tubes within the fuel bundle.
Normally the end portions of the control rod guide tubes are rigidly and unyieldingly fastened to the end pieces. These guide tubes are designed to safely resist buckling under the thrust force applied to them by the end pieces when the latter receive the force of the normal coolant flow which is upwardly against the bottom end piece and through its perforations, at least during normal reactor operations.
The pressure vessel contains a lower support structure and an upper support structure between which the fuel assemblies are held. In the event of a loss-of-coolant accident on the part of the reactor's coolant system, the coolant escaping from the reactor's pressure vessel can flow with high velocity upwardly or downwardly depending on the location of the break in the system. In either event the coolant exerts a thrust force by way of the end pieces placing the control rod guide tubes under much greater compression than they are normally required to withstand. This thrust force may be only momentary, because the emergency core cooling system is activated promptly in the event of such an accident, but it can exert sufficient compressive stress on the control rod guide tubes to cause one or more of them to buckle, this distorting the fuel rod spacer grids held by the control rod guide tubes and, consequently, dearranging the interspacing between the fuel rods themselves. This makes it difficult to control the after-cooling by the emergency water.